Primary rhesus monkey kidney (MK) cells have long been the cells of choice for isolation and propagation of the human paramyxoviruses: parainfluenza types 1, 2, 3, 4A, 4B, and mumps. In fact, all 5 human parainfluenzaviruses were first isolated in vervet or rhesus monkey kidney cells, and mumps-virus was initially recovered in rhesus monkeys. In addition to possessing a high level of paramyxovirus sensitivity, MK cells are particularly valuable because virus growth can be visualized in the cells both by cytopathology (CPE) and hemadsorption (HAd).
While parainfluenza types 2, and 3, and mumps viruses can be isolated in several human and simian cell lines, parainfluenza types 1, 4A, and 4B are generally recoverable only in MK cells. Thus, the use of MK cells for the isolation and propagation of human paramyxoviruses is generally advantageous; unfortunately, it is problematic as well. For example:
(a) There is only limited availability of rhesus monkeys because the trapping of these monkeys in India has recently been discouraged in order to preserve the species in its native habitat;
(b) Primates tend to carry many endogenous viruses ("adventitious agents"), e.g., adenoviruses, herpesviruses (including cytomegalovirus "CMV"), papovaviruses, myxoviruses, and enteroviruses, which increase the risk of infecting laboratory workers handling the animals and the animal tissues (The problem of workers contracting monkey B virus (Herpesvirus simiae), which is harmless to monkeys but is usually fatal in man, is well known.);
(c) Primary cell lines derived from the kidneys of rhesus monkeys, are sometimes latently infected with adenoviruses, enteroviruses, and herpesviruses, and almost always are infected with SV-5 (a myxovirus) and SV-40 (a papovavirus). Although these agents are not known to infect man, they often compromise virus isolation and identification, reagent production, and vaccine testing, thus constituting a major contamination problem in the laboratory;
(d) Despite the establishment of breeding colonies in the U.S., the sacrifice of monkeys for organs, such as kidneys, is generally regarded as wasteful and cruel;
(e) Monkeys are expensive to feed and house, and their use for primary cell cultures is a labor-intensive and costly endeavor.
In view of these drawbacks, the development of an alternative method for isolating and propagating human paramyxoviruses was required and sought. Until this invention, however, the alternatives suggested proved to be poor substitutes for MK cells because, inter alia, they were not as sensitive to the human paramyxoviruses as MK (e.g., Madin-Darby canine kidney ("MDCK" continuous cell line; Vero), could support the growth of no more than three serotypes of parainfluenza, and/or frequently resulted in bacterial or viral contamination (e.g., cynomolgus MK tissue culture).